Fluorescent probes can handle rigors of spaceflight

Two common fluorescent
probes may be one step closer to a flight to Mars now that researchers have determined
that the probes can withstand high doses of radiation with little degradation
of their ability to fluoresce. That is good news for scientists and engineers who
want to use these probes and others like them to hunt for life on other planets.

Little is known about how well fluorescein and
Alexa Fluor 633 would stand up to the potentially high levels of radiation involved
in deep spaceflight. So as part of a larger effort to prepare instruments for a
European Mars mission to search for life, researchers from the Space Research Centre
at the University of Leicester, from the Cranfield Biotechnology Centre at Cranfield
University, and from the University of Birmingham, all in the UK, put the fluorophores
to the test. Typically, the fluorophores are bound to antibodies and used to label
bacteria in a solution.
Researchers exposed fluorophores to cyclotron radiation to mimic the radiation of space. They used special sample blocks with 15-μl wells to hold the liquid fluorophore before evaporating the
water. Courtesy of Daniel P. Thompson.
The researchers put solutions of the fluorophores into holders specially constructed to allow multiple 15-μl wells to be put into a cyclotron beam and evaporated the water from the solution so that
only dried fluorophore remained.

“If the reagents are carried
to Mars in solution, we expect significant radiation damage to the materials,”
explained Daniel P. Thompson of the University of Leicester. “This is due
to the radiation generating chemically reactive species from the water that could
chemically attack our reagents.”

The researchers calculated the anticipated
radiation exposure for a trip to Mars during peak solar flare season and exposed
the dried fluorophores to alpha and proton radiation from the cyclotron. To ensure
that the results would cover the range of exposures, they did not account for any
spacecraft shielding and used 1, 10, 100 and 10,000 times the expected exposure.
After exposure, the fluorophores were rehydrated and tested using a Cary Eclipse
fluorescence spectrophotometer from Varian Inc. of Palo Alto, Calif. The results
showed very little degradation, even at 10,000 times the expected exposure.

Thompson said that the researchers
have conducted further experiments using gamma radiation from a cobalt-60 radionucleotide
source and that they plan to subject the fluorophores to vacuum heat testing and,
possibly, to heavier ion testing from iron, carbon, silicon or oxygen. They also
are considering tests using higher-energy protons, and they want to test other fluorophores.

To label bacteria, the fluorophores
must be bound to antibodies, so Thompson said the investigators also are testing
the effects of radiation on antibodies and plan to test the stability of antibody-fluorophore
conjugates. They hope to conduct tests during the 2007 ESA Biopan mission, which
would allow them to test the effects of multiple variables at once by sending materials
into the space environment of low Earth orbit.

Thompson said that as the design of
the spacecraft becomes clearer, the group can be more precise in its testing. However,
the results of these early tests could allow engineers to worry less about shielding
the fluorescent reagents from solar and cosmic radiation.